Delta a-photometry of open clusters

Antonia Maury (Harvard Observatory) discovered in 1897 that among the stars with about 10000 K surface temperature some were showing additional spectral lines and therefore got the denomination peculiar. Half a century later Horace Babcock found strong and stable magnetic fields on these stars and, in general, a non- coincidence of their rotational and magnetic axes. In addition, certain chemical elements concentrate at the magnetic poles. While the phenomenology of the peculiar stars turned out to be very rich, there is a persisting need for consistent models in order to understand their formation and evolution. It is the aim of this project to reach a complete census of these stars in open clusters, since they represent the relics of the star birth regions to be checked for possible hints to the formation of peculiars. That might include the size, compactness or the metallicity of the cluster. For the statistical study of the peculiar stars Delta a-photometry (established by the PI) will be carried out, because it enables their identification with a high degree of completeness and relatively short observing times. Since CCD-technology has been introduced for this 3-filter photometry the outreach in the galactic field (including the neighbouring Magellanic Clouds) has been enhanced so significantly that we are now in a position to investigate whether these peculiar stars are a global galactic phenomenon and, more specifically, how their properties depend on local circumstances.

Antonia Maury discovered in 1897 stellar spectra among hotter stars deviating from the standard pattern for a number of absorption lines. For those stars she introduced the name `peculiar` abbreviated Ap-stars. Half a century later Horace W. Babock observed extremely strong magnetic fields (1000 times stronger than the magnetic field of our Earth!) just for these peculiar stars, which also astonishingly change periodically the strength of their fields, even reversing polarity. It has been shown that in general the rotational axis does not coincide with the magnetic one, and even the magnetic axis does not cross the centre of the stars concerned. While the stars rotates a light house effect takes place with view to the observer, i.e. the positive magnetic pole area with its specific chemical enrichment alternates with the appearance of the negative pole carrying other peculiarities. In the 1950-s such a configuration was deemed physically impossible, i.e. the so called `Oblique Rotator`. Still today the formation of such a stellar scenario is unexplained. Our project has been directed to uncover the conditions in the galactic field and beyond which favour or inhibit the formation of those peculiar stars. Therefore this project concentrated on a census of Ap-stars in open clusters with the aim of reaching out substantially farther in the galactic plane than possible by spectroscopical identification techniques. This was done by the 3-filter Delta-a system sampling a characteristic flux depression in the green part of the spectrum. Has the dynamics of star formation (depending on the location in the Milky Way) an influence on the creation of those peculiar stars? We were able to refer to the first detection of bona fide peculiar stars in a neighbouring galaxy, the Large Magellanic Cloud, by this technique and performed by our local working group. Since the metallicity there is significantly smaller, the scarser appearance of peculiar stars indicated a connection between both phenomena. In line with this finding there are more peculiar/magnetic stars found closer to the galactic centre than in the antidirection. Additionally, Ap stars seem to be more present in smaller clusters. Although this finding has to be still corroborated by reducing measurements obtained in the course of the project the impression can not easily be avoided that compact star cluster formation scenarios (linked to higher metallicity) could contribute to the creation of the physically strange phenomenon of `Oblique Rotator` stars via close protostellar encounters. The study of those stars is therefore related to the earliest phases of stellar birth and therefore of prime importance including the application of the largest telescopes available.